When a temperate-climate, cold-sensitive plant is subjected to a low temperature, the plant is injured only if freezing of the intercellular liquid in the plant tissues occurs. As the plant is cooled, the intercellular liquid will supercool, unless ice crystals form on the external surface of the plant at a location where the crystals can readily penetrate into the intercellular liquid--as in the stomatal cavities, on the under surfaces of leaves and/or in the basal portions of flowers. Such ice crystals propagate into the intercellular liquid, initiating freezing therein. Such external water also supercools to temperatures as low as about -8.degree. C., unless there are present nuclei which act to initiate crystallization at a higher temperature.
Two species of bacteria, out of many species of bacteria, that commonly are present of the above-ground portions of plants--i.e., the bacteria Erwinia herbicola and Pseudomonas syringae--provide nuclei which initiate the formation of ice crystals, and cause injury to sensitive plants at relatively high temperatures--i.e., -2.degree. C. to -5.degree. C.: Lindow, S. E. et al., "The Role of Bacterial Ice Nuclei in Frost Injury to Sensitive Plants", being pages 249-263 of "Plant Cold Hardiness and Freezing Stress", Li, P. and Sakai, A., editors, Academic Press, 1978.
To protect plants from frost damage, it is therefore desirable to have available means for reducing the ice-promoting effects of these bacteria.